The accuracy of the hydrological cycle stated in the article is derived from the accuracy of its components. Globally, precipitation is poorly measured because of its high temporal and spatial variability. Simpson  states “Most of the rain in the tropic falls in regions that are relatively inaccessible to in situ measurements. The quantitative distribution over the tropical oceans is uncertain to a multiplicative factor of about two to three.” In addition, our knowledge of global evaporation is also deficient. Soil moisture, plant evapotranspiration, and the fluxes of moisture from both the oceans and land are very difficult to measure and, on a global scale, are just as poorly known as precipitation. While atmospheric water vapor can be measured globally from space, its accuracy, according to Barnett et al. , is known only to about 10–20% over the geographical oceans and 20–30% over land.
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